This invention relates to an aluminum alloy sliding bearing comprising a steel back metal and an aluminum bearing alloy, and also to a method of producing such a sliding bearing. More specifically, the present invention relates to an aluminum alloy sliding bearing which has an excellent fretting resistance to overcome the fretting of the bearing resulting from a lightweight and low-rigidity construction of a housing due to a small-size and lightweight construction and a high-speed and high-power design of a recently-developed internal combustion engine, and the present invention also relates to a method of producing such a sliding bearing. The term "fretting" means a surface damage developing when slight vibrations are present on a contact surface.
Various kinds of aluminum alloy sliding bearings are known. Generally, such an aluminum alloy sliding bearing has a bearing alloy layer (for example, of Al-Sn type, Al-Sn-Si type, Al-Zn type, or Al-Zn-Si type) formed on a surface of a back metal (JIS G3141 SPCC, SAE 1010, or the like). This back metal is usually made of low carbon steel having the carbon content of not more than 0.15%. Such low carbon steel has been used in order to reduce a deformation resistance encountered when plastically working the steel into a half-shell type configuration or a configuration with a flange.
However, in recent automobile engines, a high power output design by a high engine speed has been intended, and therefore an inertia force of a connecting rod is increased.
Therefore, in order to decrease the inertia force, the connecting rod is reduced in thickness to reduce its weight, which further reduces the rigidity of the connecting rod.
Also, there is a tendency that an engine block is made of an aluminum alloy in order to achieve a lightweight design, and as a result the engine block, like the connecting rod, has a low rigidity.
When the housing is thus reduced in rigidity, the bearing must have an increased amount of interference for mounting the bearing on the housing; otherwise the close contact between the bearing and the housing would be adversely affected at high temperatures or under a high load, which may often result in damage, such as fatigue and seizure, due to fretting or migration. The interference can be obtained by making the diameter of the bearing slightly greater than the diameter of the housing. Namely, the fixing of the bearing relative to a bore of the housing depends on such interference. Therefore, the amount of the interference is determined depending on the material, strength, etc., of the bearing and/or the housing. The term "migration" means that for example, a plating layer on the outer surface of the bearing is displaced out of place, or that foreign matters in a lubricant are moved to the outer surface of the bearing and/or the bore in the housing.
With respect to features of the conventional bearings, attention has been directed mainly to the properties of the bearing alloy, and with respect to the back metal, attention has been directed to its workability and its ability of bonding with the alloy.
However, with the high performance design of the recent automobile internal combustion engines, the total bearing performance, including not only the bearing properties of the bearing alloy but also the properties of the back metal, has now been required.
Particularly, recently, the engine tends to produce a high power output by a high engine speed, and the connecting rod tends to receive an increased inertia force due to such high engine speed.
Under the circumstances, the connecting rod tends to be reduced in thickness to reduce its weight in order to reduce the inertia force of the connecting rod, which further lowers the rigidity of the connecting rod.
Therefore, at high temperatures or at a high engine speed, the connecting rod is deformed under the influence of the heat and the inertia force because of its low rigidity. As a result, the close contact between the bearing and the housing is adversely affected, so that damage, such as fatigue and seizure, due to fretting and migration has been often encountered.
The engine block also increasingly tends to be made of an aluminum alloy in order to achieve its light-weight constriction. The aluminum alloy has a high thermal expansion coefficient of 23.times.10.sup.-6 /.degree.C., and the engine block is expanded at high temperatures, so that the close contact between the engine block with the outer surface of the metal of a sliding bearing is adversely affected, thereby causing damage due to fretting, as is the case with the connecting rod.
When the housing is thus reduced in rigidity, there is encountered damage, such as fatigue and seizure, due to fretting or migration.
In order to overcome these problems, it is necessary to increase the amount of the interference available when mounting the bearing so that the bearing can follow the deformation of the housing at high temperatures or under a high load; however, in the conventional bearing having the back metal made of low carbon steel, the strength of the back metal is low, and when the bearing is mounted on the housing under a high mounting stress, the back metal undesirably deformed beyond its elastic limit to cause permanent strain. Therefore, the intended object can not be achieved.